A Direct Current to Direct Current (DC/DC) converter is often used to improve efficiency, reduces power consumption and heat dissipation in both mobile (battery powered) and non-mobile electronic devices. A typical current mode controlled DC/DC converter includes several circuits such as an error amplifier circuit, a pulse width modulated (PWM) comparator circuit, and a current limiting circuit.
For a typical current mode controlled DC/DC converter, the error amplifier circuit monitors the converter's output voltage and reference target voltage signal and outputs the reference current signal voltage. This reference current signal voltage is converted to a current by a voltage-to-current (V/I) converter, and the PWM comparator compares this reference current to the output current. The switch control circuit turns on and off the output transistor in accordance with the output of the PWM comparator.
Additionally, the current limiting circuit is usually arranged to limit the peak value of an inductor current at the typical DC/DC converter's output, which works independently of the PWM comparator. If the inductor current is higher than a limiting value, a current limit comparator becomes high and the switch control circuit stops (turns off) the high side output transistor. In this way, Direct Current (DC) magnetic saturation can be reduced or substantially eliminated at an output inductor, which in turn protects the integrity of the converter's output transistors. However, a typical current mode controlled DC/DC converter arranged in this manner often generates relatively large output overshoot if the reference target voltage changes quickly.
In operation, the conventional DC/DC converter increases or decreases the inductor current based on at least the level of the current reference signal detected by the error amplifier. For example, if the output voltage is higher than the target voltage, the error amplifier decreases the current reference signal, then the converter would subsequently decrease the inductor current by turning off the high side output transistor earlier than the previous cycle. Alternatively, if the output voltage is determined to be lower than the target voltage, then the error amplifier would increase the current reference signal, and in response, the converter would increase the inductor current by turning off the high side output transistor later than a previous cycle.
The current limiter works independently of the PWM comparator and if the inductor current is higher than the predetermined value, the current limiter also commands the switch control circuit to turn off the high side transistor.
However there is a case, where the converter causes significant overshoot if the converter is employed above the current limit function. If the target voltage significantly changes low to high, the error amplifier increases the current reference signal and PWM comparator and switch controller increase the inductor current. When the inductor current reaches the current limit level, current limiter controls the inductor current and inductor current does not increased anymore. If the output voltage still does not reaches the target voltage, the error amplifier continues to increase it's output voltage until it hits the fixed clamp voltage. The fixed clamp voltage is usually set significantly higher voltage, and the current reference signal becomes significantly higher than inductor current, which is limited below current limit by current limit circuit. Then the output voltage reaches to the target voltage, error amplifier decreases it's output and current reference signal. However, the current reference signal is significantly higher than current limit level, it takes some time to take back the inductor current control from current limiter. During this period, the inductor current still stay around current limit value even the output is higher than target voltage. Excess inductor current lead to a significant amount of overshoot in the output voltage that can be unacceptable in many applications.